The subject of the present invention is a novel catalyst with homogeneous metals dispersion which features exceptional selectivity to desired products when employed in a hydrocarbon conversion process requiring a catalyst having both a hydrogenation-dehydrogenation function and a cracking function. More precisely, the present invention involves a novel dual-function catalyst characterized by low standard deviation of the local concentration of a Group IVA(14) metals component relative to the bulk concentration in a bed of catalyst particles which surprisingly enables substantial improvements in hydrocarbon conversion processes that have traditionally used a dual-function catalyst. Metals of Group IVA (IUPAC 14) of the Periodic Table [See Cotton and Wilkinson, Advanced Inorganic Chemistry, John Wiley & Sons (Fifth Edition, 1988)] having utility in the present invention are one or more of tin, germanium and lead.
In another aspect, the present invention comprehends improved processes that emanate from the use of the novel catalyst. In a specific aspect, an improved reforming process utilizes the subject catalyst to increase selectivity to gasoline and aromatics products. In an alternative aspect, the present catalyst is employed in a process for the dehydrogenation of dehydrogenatable hydrocarbons.
Catalysts having a hydrogenation-dehydrogenation function and a cracking function are used widely in many applications, particularly in the petroleum and petrochemical industry, to accelerate a wide spectrum of hydrocarbon-conversion reactions. The cracking function generally is thought to be associated with an acid-action material of the porous, adsorptive, refractory-oxide type which is typically utilized as the support or carrier for a heavy-metal component, such as the Group VIII(8-10) metals, to which is generally attributed the hydrogenation-dehydrogenation function.
These catalysts are used to accelerate a wide variety of hydrocarbon-conversion reactions such as dehydrogenation, hydrogenation, hydrocracking, hydrogenolysis, isomerization, desulfurization, cyclization, alkylation, polymerization, cracking, and hydroisomerization. In many cases, the commercial applications of these catalysts are in processes where more than one of these reactions are proceeding simultaneously. An example of this type of process is reforming wherein a hdydrocarbon feed stream containing paraffins and naphthenes is subjected to conditions which promote dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of paraffins and naphthenes, hydrocracking of naphthenes and paraffins and other reactions to products an octane-rich or aromatic-rich product stream. Another example is an isomerization process wherein a hdydrocarbon fraction which is relatively rich in straight-chain paraffin compounds is contacted with a dual-function catalyst to produce an output stream rich in isoparaffin compounds while converting any cyclics present to a mixture of paraffins and naphthenes by a combination of hydrogenation and ring opening. Yet another example is a hydrocracking process wherein catalysts of this type are utilized to effect selective hydrogenation and cracking of high molecular weight unsaturated materials, selective hydrocracking of high molecular weight compounds, and other reactions to produce a generally lower-boiling, more valuable output stream.
Regardless of the reactions or the particular process involved, it is of critical importance that the dual-function catalyst exhibit the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these parameters are defined as follows:
(1) Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level, with severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity typically is designated as the octane number of the pentanes and heavier ("C.sub.5 +") product stream from a given feedstock at a given severity level, or conversely as the temperature required to achieve a given octane number.
(2) Selectivity refers to the percentage yield of petrochemical aromatics or C.sub.5 + gasoline product from a given feedstock at a particular activity level.
(3) Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time or of feedstock to achieve a given C.sub.5 + product octane, with a lower rate of temperature change corresponding to better activity stability, since catalytic reforming units typically operate at relatively constant product octane. Selectivity stability is measured as the rate of decrease of C.sub.5 + product or aromatics yield per unit of time or of feedstock.
Programs to improve reforming-catalyst performance are being stimulated by the widespread removal of lead antiknock additive from gasoline and by the increasing requirements of high-performance internal-combustion engines, which magnify the requirement for gasoline "octane" or knock resistance of the gasoline component. The catalytic reforming unit must operate at higher severity in order to meet these increased octane needs. This higher severity results in lower yield of gasoline product, and catalyst selectivity becomes even more important to avoid excessive losses of valuable product to fuel by-product. The major problem facing workers in this area of the art, therefore, is to develop more selective catalysts with sufficient activity and stability to operate effectively at current high reforming severities.